In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and the DC voltage of the DC power source is converted into AC voltage by the inverter to thereby drive the AC motor.
Further, in the case where an AC motor is mounted in an electric vehicle and the like, from the viewpoint of failsafe, it is desired that an abnormality caused in a current sensor and the like can be detected. For example, in the patent document 1, in the case where the total sum of the current values of three phases sensed by current sensors disposed for three phases does not become 0, an abnormality of a current control loop is assumed to be detected.
Still further, the patent document 2 discloses the following technique: an ideal current signal is calculated by acquiring an ideal AC waveform based on a current value of one phase and is compared with three-phase current signals; and in the case where an error between them is not smaller than a specified value, the ideal current signal is assumed to be abnormal.
In the case where the current sensors are disposed for the respective phases like the patent document 1, it is thought that, for example, the drive of the AC motor is controlled on the basis of the current sensed values of two phases and an abnormality of the current sensor is detected on the basis of the current sensed value of the remaining one phase. At this time, the current sensor for controlling the drive of the AC motor (hereinafter referred to as “current sensor for controlling”) and the current sensor for detecting an abnormality of the current sensor (hereinafter referred to as “current sensor for monitoring”) need to be clearly differentiated from each other. This is because of the following reason: in a system the current sensed value is fed back in such a way as to become equal to a desired sine wave current (for example, the main motor of an electric vehicle), all current sensed values sensed by the current sensors for controlling are controlled precisely in such a way as to become a desired sine wave current, so that when the current sensed value of the current sensor for controlling is used for the abnormality of the current sensor, it looks like as if an abnormality were not caused in the current sensor. This is referred to as “the interference of current feedback”.
In order to prevent the interference of current feedback, at least at the time of detecting an abnormality of the current sensor, the current sensor for monitoring needs to be entirely independent of a feedback loop. To this end, the current sensor for controlling, which is provided for the purpose of controlling the drive of an AC motor, needs to be clearly differentiated from the current sensor for monitoring. In this case, even in a system in which high-speed sampling and high-speed feedback control are performed, a detected value for detecting an abnormality of the current sensor for monitoring is outside the current feedback control and hence is allowed to be detected at a lower sampling speed without being restricted by the control speed. For example, the sampling period of the current sensor for controlling, which is used for the current feedback control, is several hundreds μsec, whereas the sampling period of the current sensor for monitoring is several msec.
On the other hand, even if the sampling speed of the current sensor for monitoring is allowed to be low, seeing that an abnormality of the current sensor is detected by the comparison or combination of the sensed value of the current sensor for monitoring and the sensed value of the current sensor for controlling, if the current sensor for controlling and the current sensor for monitoring are different from each other in performance such as detection accuracy, failure rate, and temperature characteristics, the detection accuracy of an abnormality is likely to be impaired. For this reason, there is no alternative but to use the current sensor for monitoring of the same quality as the current sensor for controlling, so that the current sensor for monitoring and the current sensor for controlling are equal to each other in size and cost. Hence, this prevents the request of reducing size and cost from being satisfied.
Further, in the patent document 2, by the use of the AC waveforms of respective phases being ideally shifted by 120°, one-phase current value is delayed by 120° in such a way as to correspond with the ideal AC waveform, whereby the current values of the other phases is estimated. The patent document 2 is a technology of questioning only a gain error between two current sensors and of focusing attention on the fact that a gain error cannot be caused by shifting only a phase in the current sensed value of one phase. An ideal current signal acquired by this method is not information reflecting actual respective currents with high accuracy and hence an abnormality of the current sensor cannot be suitably detected. Further, in the detection of an abnormality in the patent document 2, the interference of current feedback is not considered. Further, at least at the time of detecting an abnormality, it is necessary to bring about the state where the current sensor for controlling is clearly differentiated from the current sensor for monitoring. However, the patent document 2 makes no reference to these points and hence, according to the technique of the patent document 2, it is highly probable that an abnormality of the current sensor cannot be suitably detected.    [Patent document 1] Japanese Unexamined Patent Publication No. Hei 06-253585    [Patent document 2] Japanese Patent No. 4942425